1. Field of the Invention
The present invention relates to a method for monitoring a radio-frequency transmitter device of a magnetic resonance tomography system, having a transmit antenna system that has a multiplicity of transmitter channels, during a magnetic resonance measurement of an examination subject. In addition, the present invention relates to a radio-frequency transmit device for such a magnetic resonance tomography system, to a radio-frequency monitoring device for such a radio-frequency transmit device, and to a magnetic resonance tomography system having such a radio-frequency transmitter device.
2. Description of the Prior Art
An imaging using a magnetic resonance tomography system essentially takes place in three steps. First, a strong, stable, homogenous magnetic field, and thus a stable orientation of the protons in the relevant body region, is produced around the body region. This stable orientation is then modified by electromagnetically supplying radio-frequency energy. Third, this energetic stimulation is then terminated and the nuclear resonance signals resulting in the body are measured (detected) using suitable receive coils, in order to permit conclusions to be drawn concerning the tissue in this bodily region. For this purpose, a magnetic resonance tomography system has a large number of components that work together, each of which requires the use of modern, expensive technologies. A central element of a magnetic resonance tomography system, to which the present invention also relates, is the radio-frequency transmitter device, which is responsible for the generation of the radio-frequency pulses that are to be radiated into a body region. The radio-frequency pulses emitted by a radio-frequency power amplifier of the radio-frequency transmitter device are conducted to a transmitter antenna system that radiates the radio-frequency pulses into a bodily region.
In the development and establishment of magnetic resonance tomography systems, in order to ensure patient safety boundary values have been standardized that regulate the maximum radio-frequency radiation into a human body. A typical boundary value is the maximum permissible SAR (Specific Absorption Rate) value. For example, for the valve known as the whole-body SAR it is required that the power absorbed by the patient in a time window averaged over 6 minutes must not exceed a value of 4 W/kg. Measurement devices that can be used to measure the radio-frequency power are provided in the magnetic resonance systems. It is standard for this purpose to use directional couplers in the leads to the antenna system. Because these directional couplers are able to measure only the total power, they always also acquire the power that is converted into lost heat in the apparatus itself, for example in the cables or in the transmitter antenna systems. In order to enable maximum exploitation of the permissible boundary values, and thus to improve the imaging overall and/or to shorten the measurement time, it would be desirable to be able to determine the portion of the transmit power remaining in the transmit antenna system, or to eliminate it from the measurement result. This makes sense in particular if the load on the transmitter antenna system due to the patient is only slight, which is the case, for example, in examinations of the head alone.
For magnetic resonance systems having simple, linearly or circularly polarized antennas, suitable methods are known for determining the antenna losses. However, these methods cannot be transferred directly to transmitter antenna systems having a number of independent transmitter channels, for example to transmitter antenna systems that have a birdcage antenna, in which the individual antenna rods are separately controllable for the production of arbitrary field distributions. In such transmitter antenna systems, in contrast to purely linearly or circularly polarized antennas, the current distribution changes according to the selected controlling, so that as a consequence the losses remaining in the transmitter antenna system can also vary strongly.